The present invention relates to ballast circuits for high intensity discharge lamps or any arc lamps (e.g., flourescent) and, more specifically, to lead ballasts for use with such lamps.
High intensity discharge lamps are popularly used because of their efficiency and daylight-spectrum light capability. Such lamps usually include a partially evacuated tube containing a gas such as sodium vapor. Two electrodes within the tube apply a voltage across the gas and thereby ionize it. The ionized gas glows as it conducts current. Lamps of this type initially have a very high impedance (before the gas ionizes) and require a high start up voltage. However, once the lamp has ignited, i.e. the gas has ionized, the voltage across the lamp drops sharply during a warm-up period. As the lamp warms up, during a three to ten minute period, its voltage rises until it reaches a stable operating level.
As a result of the voltage-current characteristics of these lamps, it is not practical to connect them directly to a voltage source, since the current through the lamps would damage the lamps and/or the power source. Consequently, circuitry is needed which provides a high ignition voltage to the lamp and which also limits the current through the lamp during its warm-up period.
Other characteristics of discharge lamps should also be considered when providing a lamp circuit. For example, lamp life is prolonged if the crest factor, or ratio of the maximum versus the average current applied to the lamp, is kept low. In addition, as the lamp ages, its voltage increases. The current supplied to the lamp should then decrease to maintain a constant power level to the lamp. To maintain efficiency of the system, the power factor of the power supplied to the lamp should be as near unity as possible. Finally, the lamp circuit should compensate for variations in supply voltage.
Various types of current-limiting circuits, or ballasts, are well known in the art for use with high intensity discharge lamps. The ballasts currently known include one resistive type and two reactive types. The reactive types are further classified according to whether they are lead or lag ballasts, i.e., whether they cause the current to lead or lag the voltage.
A resistive ballast circuit comprises a resistor in series with the lamp and the power supply so that the resistance of the ballast limits the current through the lamp during the lamp warm-up period. However, these ballasts dissipate a considerable amount of energy during normal operation of the lamp and consequently are not suitable for use in many cases.
The lag ballast usually comprises an inductor in series with the lamp circuit and a capacitor connected, for power factor correction, across the lamp-inductor combination. The term lag refers to the predominantly inductive nature of the lamp circuit. More specifically, in operation, the lag ballast inductance limits current through the lamp without, itself, dissipating much power. For normal operating conditions the lag ballast impedance is usually approximately equal to the lamp impedance and the supply voltage is equally distributed between them.
A problem inherent in lag ballasts occurs during warmup, when the lamp impedance decreases and most of the supply voltage is across the ballast. The current through the ballast now increases and may cause it to saturate, thereby allowing a further current increase which may be harmful to the lamp, ballast, and power supply. Thus, the lag ballast must be physically large to prevent saturation during warmup. Further, the reactance of the ballast must be high to adequately limit the lamp current during warm up.
A typical lead ballast currently known in the art includes an inductor and capacitor which are both in series with the lamp and the power source. The impedance of the ballast in this case is the difference between the inductive reactance and the capacitive reactance. The high current during the initial warm-up period saturates the inductor and the series reactance of the inductor and capacitor combination thus increases to limit the current through the lamp. After the lamp warms-up and the inductor unsaturates, the impedance of the ballast decreases but remains considerably higher than the lamp impedance.
There are several problems associated with conventional lead ballasts. In particular, the ballast tends to function as a constant current source which maintains an essentially constant current through the lamp despite lamp voltage variations due to ageing. Consequently, the power to the lamp increases as the lamp ages which shortens its life and changes the color of its light.
The inductor of a lead ballast may be a transformer secondary winding which increases the voltage across the lamp during warm-up. Unfortunately, this arrangement multiplies any changes in the power supply voltage variation and amplifies changes in lamp current which, in turn, shortens the lamp life.